Polyethylene has been modified and synthesized in situ by a combination of Cp2ZrCl2 and 8-aminoquinoline nickel dichloride catalysts with methylaluminoxane co-catalyst. The results indicate that the crystallinity and Tm of polyethylene both decrease with increasing Ni/Zr mol ratio when the Ni catalyst is added first. Oligomers formed by 8-aminoquinoline nickel dichloride within a certain Ni/Zr mol ratio could be completely converted into polymer. Three methods used to introduce catalysts to the reaction system resulted in different products. Polyethylene with the lowest melting point, crystallinity and the highest degree of branching resulted when the Zr catalyst was added after that of the Ni compound.© 2001 Society of Chemical Industry